The foreign secretary compared the Irish border to the Camden/Westminster boundary.

The Tories used to regularly accuse Jeremy Corbyn and his key lieutenants of only being interested in liberal metropolitan London.

Now Boris Johnson has dramatically opened himself up to the charge – by comparing the border between Northern Ireland and the Republic to the boundary between the London boroughs of Camden and Westminster.

The foreign secretary said the comparison worked because money was "invisibly" taken from people travelling between Camden and Westminster when he was London mayor.

But - of course - his remarks were immediately seized on by pro-Europeans, with Labour MP Chris Leslie leading the charge on behalf of the pro-EU campaign Open Britain.

He said: "To compare the border between two sovereign states, the UK and the Republic of Ireland, to the boundaries between different London boroughs is not only patently ridiculous but also shows staggering insensitivity and a stupefying ignorance of a conflict in which over 3,000 people died between 1969 and the signing of the Good Friday Agreement."

Meanwhile on Twitter, the Blairite former cabinet minister Lord Adonis had a more pithy riposte: "What planet?"

But perhaps the sharpest rebuke was delivered by journalist Iain Martin who summarised the latest development: "The Boris solution to the Irish border seems to be... the Oyster card. Unbelievable."

The foreign secretary had told Radio 4’s Today programme: "We think that we can have very efficient facilitation systems to make sure that there's no need for a hard border, excessive checks at the frontier between Northern Ireland and the Republic.

"There's no border between Islington or Camden and Westminster... but when I was mayor of London we anaesthetically and invisibly took hundreds of millions of pounds from the accounts of people travelling between those two boroughs without any need for border checks whatever."

He then insisted: "It's a very relevant comparison because there's all sorts of scope for pre-booking, electronic checks, all sorts of things that you can do to obviate the need for a hard border to allow us to come out of the customs union, take back control of our trade policy and do trade deals."